http://social.microsoft.com/forums/en-US/onecareinstallandactivate/thread/399071e2-49dc-4767-932d-d261e3965943/If that doesn't resolve it, see this KB article:
http://support.microsoft.com/kb/930162And/or contact support for help.
How to reach support (FAQ) - http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90Why did you uninstall OneCare?-steve